Inductors are often used in integrated circuits, such as the voltage controlled oscillator 100 shown in FIG. 1. When multiple inductors L1 and L2 are present in such circuits, or in separate circuits on the same IC substrate, there is a risk that the inductors magnetically couple with each other, which, in turn, may affect the operation of the integrated circuit as the resulting currents induced in the components can cause unwanted changes in their behavioural characteristics. The location and proximity of these components is a factor in the degree of magnetic coupling present. To mitigate this problem, integrated circuits are often designed such that inductors are physically separated as far as is practical. However, such design topologies occupy a large area on chip and it is desirable to minimise the chip area required for an integrated circuit. Furthermore, it is desirable to conserve chip area without compromising the performance of the integrated circuit.
It has been proposed to reduce the area required by a circuit comprising more than one inductor by embedding an inductor within another. A design of an integrated inductor and transformer known in the art is illustrated in FIG. 2 (from US 2011/0248809 and US2012/0326826) where the inductor structure 200 includes a first inductor 201 and a transformer 202 comprising a second inductor 203 and a third inductor 204, in which the first inductor 201 is embedded within the transformer 202, such that the magnetic effect of a current flowing through the inductor 201 cancels that of the outer inductors 203 and 204 such that no magnetic coupling of these coils occurs. This cancellation of magnetic effect in FIG. 2 is due to the figure-of-eight configuration of the first inductor 201 such that the magnetic component generated by the current flowing in outer inductors 203 and 204 is removed, while at the same time said outer inductors 203 and 204 are interleaved to form the transformer 202.
A low noise amplifier (LNA) circuit 300 using the inductor structure 200 is shown in FIG. 3. The circuit 300 comprises several elements and includes inductor elements 201, 203 and 204. The circuit 300 shows how the first inductor 201 and outer inductors 203 and 204 of inductor structure 200 can be connected. It is clear from FIG. 3 that while the inductor structure 200 of FIG. 2 economises on chip area, the device does not have the functionality to operate all inductors as discrete isolated inductors that can be configured to operate independently or together as coils L2 and L3 cannot be decoupled from transformer 202 and used separately. Further, the outer inductors 203 and 204 (represented by coils L2 and L3) and the inner inductor 201 (represented by coil L1) do not share a common ground connection which balances the whole structure in which a node common to all of the coils is forced to specific potential where the common mode current through coils L1 to L3 can be controlled.
There is therefore a need for an improved integrated inductor structure that can be configured to operate as independent inductors, or as a composite inductor, as required by an integrated circuit, while minimising the occupied chip area and ensuring mutual isolation between the independent devices.